In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Radically polymerizable cyclic monomers are of particular interest due to their much lower polymerization shrinkage compared with linear monomers (cf. R. K. Sadhir, R. M. Luck, Expanding Monomers, CRC Press, Boca Raton etc. 1992).
WO 96/19471 discloses mono- or difunctional cyclic allyl sulphides which are said to be suitable for the preparation of adhesives, dental composites and optical lenses. These monomers are said to display a lower shrinkage during polymerization than conventional monomers. Unfortunately, compared with commercial dimethacrylates, these monomers are characterized by an unsatisfactory, too low reactivity during radical photopolymerization.
R. A. Evans, E. Rizzardo, Macromolecules 29 (1996) 6983-6989, describe the ring-opening polymerization of 6-methylene-1,3-dithiepane and 3-methylene-1,5-dithiacyclooctane. These monomers are said to be resistant to moisture and also acids and bases, but have a penetrating, unpleasant smell.
In later works, R. A. Evans, E. Rizzardo, Macromolecules 33 (2000) 6722-6731 and R. A. Evans, E. Rizzardo, J. Polym. Sci.: Part A Polym. Chem. 39 (2001) 202-215, the same authors investigate the influence of substituents on the polymerization of mono- and bicyclic allyl sulphides. In the case of unsymmetrically substituted monomers, amorphous polymerization products were obtained which are said to be characterized by a lower shrinkage compared with crystalline products. The investigated bicyclic monomer bis(6-methylene-1,4-dithiacycloheptane-2-ylmethyl)diglycolate produced a rubbery, soft polymer which is unsuitable for dental applications.
W. Weinmann, C. Thalacker, R. Guggenberger, Dent. Mat. 21 (2005) 68-74, describe dental materials based on cyclosiloxanes modified with cycloaliphatic epoxides, so-called siloranes, which are said to make possible a further reduction in the polymerization shrinkage. However, a disadvantage of these compounds is their sensitivity to acids.